The Third Generation Partnership Project (3GPP) wideband code division multiple access (WCDMA) initial release (R99) includes mechanisms for soft combining on the downlink for the dedicated channels (DCH). In soft combining operations, the wireless transmit/receive unit (WTRU) receives the same information via multiple Node Bs and combines the received information at the soft bit level. This was possible due to the constant over-the-air bit rate that was transmitted simultaneously across all Node Bs. When high-speed downlink packet access (HSDPA) was introduced in Release 5, this approach could no longer work in this context, because the instantaneous bit rate in HSDPA is determined locally at each Node B based on instantaneous channel measurements. The throughput increase obtained by using instantaneous channel measurements surpassed the macro-diversity gain obtained by soft combining.
More recently, the WCDMA standards in Release 8 introduced dual-cell HSDPA operations (DC-HSDPA), where the WTRU receives data simultaneously from two cells of the same Node B over adjacent frequencies in the same radio band. This approach allows doubling the WTRU downlink data rate (while also using double the bandwidth). In Release 9 and Release 10 of the standards, the concept was extended to support multi-band operations and up to four simultaneous downlink carriers. While this approach improves the WTRU throughput across the cell, it does so at the expense of additional bandwidth and does not provide significant system-wide gain. For WTRUs experiencing cell-edge conditions, other techniques may provide improved coverage while not necessitating the additional bandwidth.
Other approaches have been proposed to take advantage of the presence of the second or multi-receiver chain (necessary for multi-cell HSDPA, e.g., 2C/4C HSDPA, operations) to receive over at least two different cells, but in the same frequency to improve the reception throughput at the cell edge or the sector edge, potentially increasing spectral efficiency. This gain may be realized by using multipoint (or multi-cell) transmission/reception of data from geographically separated cells (points) in the same frequency and/or different frequencies. This form of operation is referred to as multipoint HSDPA operation. It is noted that single-frequency DC-HSDPA (SF-DC-HSDPA) is one example embodiment of multipoint HSDPA.
Approaches to provide throughput gains for multipoint HSDPA may be loosely grouped into four different categories (source switching, soft combining, source multiplexing, or multi-flow aggregation) based on the number of different transport blocks the WTRU may receive at each transmission time interval (TTI). In source switching, the WTRU receives data from a single source at a time, but may receive data from multiple sources over time. In soft combining, the WTRU receives the same data from multiple sources and combines the soft information for improved detection performance. In source multiplexing, the WTRU receives different data from multiple sources simultaneously. All of these approaches attempt to improve WTRU throughput at the cell edge or the sector edge.
Depending on the mode of the multipoint DC-HSDPA operations, the WTRU has to perform a number of tasks to demodulate the data carried on the high speed physical downlink shared channel (HS-PDSCH). To do so at a reasonable complexity, it is preferable for the Node B to transmit basic information to help the WTRU decide what part of the code space to decode, and how it is modulated and encoded in general. This signaling and the associated WTRU actions may take different forms, depending on the multipoint HSDPA mode of operation.